Conventional golf balls can be divided into two general classes: solid and wound. Solid golf balls include one-piece, two-piece (i.e., single layer core and single layer cover), and multi-layer (i.e., solid core of one or more layers and/or a cover of one or more layers) golf balls. Wound golf balls typically include a solid, hollow, or fluid-filled center, surrounded by a tensioned elastomeric material, and a cover.
Examples of golf ball materials range from rubber materials, such as balata, styrene butadiene, polybutadiene, or polyisoprene, to thermoplastic or thermoset resins such as ionomers, polyolefins, polyamides, polyesters, polyurethanes, polyureas and/or polyurethane/polyurea hybrids, and blends thereof. Typically, outer layers are formed about the spherical outer surface of an innermost golf ball layer via compression molding, casting, or injection molding.
From the perspective of a golf ball manufacturer, it is desirable to have materials exhibiting a wide range of properties, such as resilience, durability, spin, and “feel,” because this enables the manufacturer to make and sell golf balls suited to differing levels of ability and/or preferences. In this regard, playing characteristics of golf balls, such as spin, feel, CoR and compression can be tailored by varying the properties of the golf ball materials and/or adding additional golf ball layers such as at least one intermediate layer disposed between the cover and the core. Intermediate layers can be of solid construction, and have also been formed of a tensioned elastomeric winding. The difference in play characteristics resulting from these different types of constructions can be quite significant.
A known problem with golf balls is that water vapor sometimes penetrates into golf ball materials, which can harmfully affect golf ball properties. For example, when a polybutadiene core cross-linked with peroxide and/or zinc diacrylate absorbs water, the core tends to lose resiliency, and the compression and coefficient of restitution (CoR) of the ball may change.
Typically, at 38° C. and 90% humidity over a sixty day period, significant amounts of moisture can enter the cores and reduce the initial velocity of the balls by 1.8 ft/s to 4.0 ft/s or greater. The change in compression may vary from about 5 PGA to about 10 PGA or greater. The absorbed water vapor also reduces the golf ball CoR. When a golf ball is subjected to prolonged storage and/or use under ambient conditions such as 25-35% RH, as well as conditions of high temperature and high humidity, the CoR of the golf ball tends to decrease over time due to water vapor absorption. Unfortunately, at least some widely used polyurethane cover materials also tend to be vulnerable to moisture penetration, and therefore can't adequately protect the core.
The industry has addressed such problems by applying a moisture barrier layer over a golf ball material that would otherwise be vulnerable to water penetration. In this regard, an effective moisture barrier layer has a moisture vapor transmission rate (MVTR) that is low enough to create a barrier against moisture penetration into the enveloped material and thereby protect the material against the negative effects of water.
It has been determined that in many golf balls, the moisture barrier layer should be as thin as is possible in order to avoid compromising other important golf ball properties such as CoR, durability, and compression or unnecessarily increasing manufacturing costs. To date, injection molding very thin layers of moisture barrier material has undesirably tended to create non-concentric, non-conformal layers containing pin holes and having non-uniform thicknesses, each which can produce undesirable performance characteristics and/or present durability issues.
Accordingly, coating layers of moisture barrier material have also been explored, but can present layer thickness uniformity issues, as well as adhesion and durability problems when air pockets formed at an interface between the coating material and adjacent inner layer. In this regard, prior thin moisture barrier coating layers of nano-composite filled elastomeric coatings containing exfoliated platelet particles or a specialized low transmission polymer such as polyvinylidene chloride (“PVDC”) have displayed inter-layer adhesion problems, durability issues, resilience deficiency, and loss of barrier effectiveness when the moisture barrier coating layer cracks due to impact by a golf club. The barrier properties of PVDC coatings are typically best at or below ambient (room) temperature (˜68-77° F.), but degrade rapidly at elevated temperatures.
Thus, golf ball manufacturers have instead tried incorporating thin films of moisture barrier material in golf ball constructions. For example, very thin metallic film layers have been incorporated for this purpose. See, e.g., U.S. Pat. No. 9,433,826 of Comeau et al. However, a catalytic coating pre-treatment is generally needed in connection with such films, which can complicate as well as add to the cost of golf ball manufacture. In a different approach, flat ionomer resin film sheets having a thicknesses of from 10 microns (um) to less than 300 um were compression molded about a ball body while dimples were also being formed in the resin. However, there was no way to prevent the resin sheets of moisture barrier material from overlapping in some areas on the ball body surface during the heat compression molding process, which tended therefore to produce non-uniformity in the resulting film layer and adhesion issues between the film and encased ball body at an interface there between.
In an alternative approach, very thin ionomer sheet blanks were vacuum-suctioned into and within inner faces of concave hemispherical cup-shaped half-shell molds. Pre-formed half shells of thin moisture barrier film were thereby produced having a thickness of no greater than 1.5 mm, but durability issues could arise because this method permitted the outer diameter and shape of the resulting pre-formed cup-shaped half-shell films to differ from the diameter and shape of the inner face of the mold which could result in gaps between the cup-shaped films and an encased core/subassembly since the contour of the film pre-form did not always match the contour of the encased core/subassembly.
Meanwhile, a major challenge encountered with incorporating thin film moisture barrier layers in golf ball constructions is achieving excellent normalized MVTR (nMVTR) criteria while still producing a material able to withstand the degree of elongation sufficient for sustaining the tremendous force of a golf club face striking the golf ball. Normalized MVTRs compare the ability of materials to resist moisture penetration irrespective of the thickness of the material and can be determined by the equation VTR(g·mm/m2·day)·(1/thickness (mm)) or g/(m2·day). For example, ionomers typically have an nMVTR of from 9 to 12 g/(m2·day), which is better than that of thermoset and thermoplastic polyurethanes, but still not ideal. And while polyvinylidene chloride coatings can demonstrate nMVTRs in the range of 3.9 to 6.3 g/(m2·day), such coatings generally have the limitations of coatings discussed above.
In this regard, related U.S. application Ser. No. 15/466,916 of Hogge discloses golf balls incorporating at least one very thin thermoformed moisture barrier film layer having an nMVTR of less than 4.0 g/(m2·day) and a continuous and substantially uniform thermoformed thickness. First and second heat-induced pre-form half shells have first and second inner surfaces that are sized and shaped and contoured to receive and conformally and adhesively mate onto and about the subassembly—which results in a finished golf ball wherein the very thin thermoformed moisture barrier film layer has a uniform thickness and excellent adhesion with adjacent inner and outer conventional golf ball layers.
However, there is still a need for golf balls including more flexible moisture barrier films that have comparably low nMVTRs yet can be incorporated in a wide range of golf ball constructions durably via any of vacuum forming, compression molding, and/or thermoforming—without meanwhile changing important characteristics of the golf ball. Such golf balls and methods of making the golf balls would be useful and desirably cost effective. Golf balls of the invention and methods of making golf balls of the invention address and solve these needs.